T cell activation via TCR ligation and CD28 costimulation results in the nuclear translocation of several transcription factors, an event thought necessary for cytokine production, proliferation, survival etc. In particular, CD28 costimulation markedly potentiates TCR-induced activation of NF-kB. However, whether NF-kB is in fact necessary for CD28 and TCR-mediated functions in vitro and in vivo is not fully understood. The global aim of this study is to determine whether activation of NF-kB is necessary for defined T cell functions driven by ligation of TCR and CD28. Specific Aim 1. To determine whether activation of NF-kB is necessary and/or sufficient for CD28-mediated costimulation of T cells in vitro. We hypothesize that NF-kB activation is selectively required for a subset of T cell functions promoted by CD28 costimulation. To address this issue, T cells from 2 strains of mice deficient in NF-kB activation will be utilized. To address whether NF-kB activation is sufficient to bypass a need for CD28 costimulation, T cells from CAR transgenic mice will be transduced with an adenoviral construct expressing a constitutively active form of IKKb. In all cases, effects on T cell survival, cytokine production, and differentiation into effector cells will be examined. Specific Aim 2. To investigate whether lack of NF-kB activation prevents the lymphoproliferative disease observed in CTLA-4-deficient mice. CTLA-4, a molecule with marked homology with CD28, is a negative regulator of T cell responses that also inhibits NF-kB activation. We hypothesize that the lymphoproliferation occurring in CTLA-4-deficient mice is due, in part, to unopposed NF-kB activation in T cells. To address this possibility, CTLA-4-deficient mice will be interbred with mice expressing the IkBa transdominant gene on T cells, and life span, numbers of T cells in peripheral lymphoid organs and in solid organs, and T cell function will be monitored. Specific Aim 3. To determine whether NF-kB activation is necessary for acute allograft rejection. To address the requirement for NF-kB activation in acute allograft rejection, skin and heart allografts will be transplanted into mice with deficient NF-kB activation in T cells. Graft survival, T cell priming, migration, and effector function will be analyzed.